1. Field of the Invention
The present invention pertains, in general, to a catalyst for selective catalytic reduction of nitrogen oxides and a method for preparing the same. More specifically, the present invention pertains to a preparation of a catalyst for selective catalytic reduction of nitrogen oxides having excellent removal efficiency of nitrogen oxides, a thermal stability at high temperatures, and chemical stability and poisoning resistance to various inorganic, organic dusts and sulfur compounds contained in an exhaust gas, which is prepared by recycling an alumina-based spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
2. Description of the Prior Art
Generally, nitrogen oxides (NOx) are inevitably generated by plants which consume fossil fuels, such as power plants and chemical plants. Nitrogen oxides are found to be an immediate cause of the pollution, such as acid rain and smog. Now, most countries including Korea strictly forbid the discharge of nitrogen oxides above the allowed standard levels. Accordingly, a technique for removing nitrogen oxides from waste gas in a combustion system has been devised.
Meanwhile, to eliminate the source of nitrogen oxides emissions, which are produced by a reaction of nitrogen and oxygen in the presence of excess air in high temperature combustion equipment, there have been made many attempts for the improvement of the combustion conditions, such as low oxygen combustion and exhaust gas circulation. However, the nitrogen oxides cannot be completely eliminated only by improvements in combustion technique and thus, there are developed and suggested various post-treatment techniques by which the exhaust gas may be deprived of nitrogen oxides.
Techniques for effectively eliminating nitrogen oxides (NOx) are commonly classified into a selective catalytic reduction (SCR) using a catalyst and a reductant together, a selective non-catalytic reduction (SNCR) using only a reductant without a catalyst, a low-NOx burner technique controlling a combustion state in the burner and so on. Among them, the selective catalytic reduction is valued as an effective technique for removing nitrogen oxides, taking notice of the generation of secondary pollution, removal efficiency, operation cost, etc. By using the selective catalytic reduction technique, nitrogen oxides may be removed with an efficiency of 90% or greater and the endurance period thereof may be used for about 2-5 years. In addition, said technique is technically advantageous because poisonous dioxin may be removed, along with nitrogen oxides, in the incinerator.
Catalysts useful in the selective catalytic reduction are classified into an extruded honeycomb catalyst, a metal plate catalyst, and a pellet catalyst, depending on their external forms. Currently, the extruded honeycomb and the metal plate catalysts are widely used in steam powder plants and incinerators. Useful as a support of the catalysts are titania, alumina, silica, zirconia and so on, and the catalyst composition mainly comprises oxides of active metals such as vanadium, molybdenum, nickel, tungsten, iron, and copper, and further comprises other active metal components for broadening temperature ranges and enhancing durability of the catalyst.
It became recently known that a catalyst for selective catalytic reduction can be manufactured containing oxides of crystalline phases by impregnating a support of inorganic oxides such as titania, alumina, silica and zirconia with catalytic components such as vanadium, molybdenum, nickel and tungsten, followed by thermal treatment.
In this regard, U.S. Pat. No. 5,827,489 discloses a process for the preparation of a catalyst for selective catalytic reduction containing oxides of crystal phases by impregnating a support of inorganic oxides such as titania, alumina, silica and zirconia with catalytic components such as vanadium, molybdenum, nickel and tungsten, thereafter heat treating. This patent employs a support and catalytic components with a superior poisoning resistance to sulfur oxides for the selective catalytic reduction and has advantages of freely controlling the amounts of active metals, a specific surface area and pore sizes of the catalyst to prepare the catalyst having optimal performance in which a suitable amount of sulfate is added. On the other hand, it suffers from high preparation cost because each of single materials (or precursors) used as the support and the catalyst should be prepared by methods of catalyst production and mixing.
Meanwhile, oil refineries essentially employ a hydro-desulfurization process for removing sulfur components contained in crude oil, from which a spent catalyst is discharged as a by-product. However, if such a spent catalyst is not recycled, treatment cost therefore are required continuously, which is disadvantageous in the economic aspect:
In this regard, Korean Patent Laid-Open No. 95-72277 and U.S. Pat. No. 6,171,566 refer to recycling of spent catalysts discharged from a hydro-desulfurization process of an oil refinery. A catalyst for selective catalytic reduction prepared by recycling such spent catalysts is more advantageous in terms of low preparation cost, inherent poisoning resistance to sulfur oxides, and containing the high content of metal components with excellent activities for nitrogen oxides reduction, compared with a catalyst prepared by a combination process of single materials.
However, when a spent catalyst is used alone in the selective catalytic reduction of nitrogen oxides, the active metal components contained therein are not uniformly impregnated within pores of support, or on its surface, but present in lump form, thereby lowering catalytic performance. The above patent suffers from the disadvantages that catalytically active components are impregnated at excessive levels rather than required. Further, a small amount of other metal components and an excess amount of sulfur compounds which decreases catalytic performance are present, and also the spent catalyst discharged from different discharge-lines after a hydro-desulfurization process has different properties, and thus it is difficult to apply to commercial catalytic processes. In the case of using the pretreated spent catalyst alone in the preparation of a catalyst for selective catalytic reduction of nitrogen oxides, thusly prepared catalyst does not exhibit sufficiently satisfactory performance. Also, because the support in the spent catalyst mainly comprises alumina, poisoning may occur by a physical adsorption or a chemical reaction when the catalyst is employed in the application in which sulfur components and oil sludge are discharged at a large amount.
Leading to the present invention, the intensive and thorough research on a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding of a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides in the form of a catalyst body having excellent performance and durability, and low preparation cost from the spent catalyst.
Therefore, it is an object of the present invention to provide a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides, which has excellent removal effect of nitrogen oxides and poisoning resistance.
It is another object of the present invention to provide a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides by recycling a spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
It is a further object of the present invention to provide a catalyst for selective catalytic reduction of nitrogen oxides prepared by recycling a spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
In accordance with the present invention, there is provided a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides comprising the following steps:
a) pretreating a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, comprising 1 to 30 wt % of vanadium, 1 to 20 wt % of nickel, 1 to 20 wt % of molybdenum and 1 to 15 wt % of sulfur component on alumina, by thermally treating said spent catalyst followed by washing with water
b) providing a support impregnated with 1 to 15 wt % of tungsten on the support basis, said support being selected from the group consisting of alumina, titania, silica, zeolite and a mixture thereof;
c) pulverizing the pretreated spent catalyst, followed by homogeneously wet-mixing the pulverized spent catalyst with the tungsten-impregnated support under the addition of water and acid;
d) dehydrating the mixture to remove excess moisture and non-impregnated active metals therein;
e) drying the dehydrated mixture, followed by grinding the dried mixture; and
f) extruding the grinded mixture or coating the grinded mixture to a structure, followed by drying and then calcining to form a catalyst body.